Commitments and Contingencies, Variable Interest Entities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($) MW
Purchased Power Agreements [Abstract]
VIE Purchase Power Agreement Expiration	2033
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW) | MW	3,698	3,698
Harrington Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration	2016
Tolk Station Power Plant
Fuel Contracts [Abstract]
Coal Supply Agreement Expiration	2017
Low-Income Housing Limited Partnerships
Amount Reflected in Consolidated Balance Sheets [Abstract]
Current assets	$ 6,274	$ 6,609
Property, plant and equipment, net	51,480	53,047
Other noncurrent assets	1,394	1,503
Total assets	59,148	61,159
Current liabilities	7,540	7,774
Mortgages and other long-term debt payable	31,082	31,207
Other noncurrent liabilities	644	619
Total liabilities	$ 39,266	$ 39,600